The results of a human study that researchers at the US Army Research Institute of Environmental Medicine will soon publish in Medicine & Science in Sports & Exercise fly in the face of the message found in nearly all popular articles on exercise, health and hormones. But because we know that our readers weren’t born yesterday, we’ve written about it. According to the researchers, two hours of cardio training boosts your body’s growth hormone secretion more than doing one or two hours of strength training.
Ways of optimising your endogenous production of growth hormone are of particular interest to life extenders because growth hormone helps to restore muscle fibre, bone, cartilage and skin. But growth hormone is also of interest to people who want to lose fat. Growth hormone pulses stimulate muscle cells to burn fat.
Intensive exercise stimulates the secretion of growth hormone, according to many studies. But most of these only measure growth hormone secretion during the first few hours after an exercise session. No studies had ever been done that measured the effect over a 24-hour period, and that compared the effect of different kinds of exercise. So these researchers decided to fill this gap.
The researchers got eight male recreational athletes who were in their twenties to do on different occasions: a) nothing [Control], b) an hour of strength training using weights with which they could manage 5-10 reps [Moderate strength training], c) two hours of strength training [Long strength training], d) an hour of cycling at 70 percent of VO2max [Moderate aerobic training] or e) two hours of cycling [Long aerobic training].
The strength training session consisted of squats, bench presses, leg presses and lat pull-downs; the cardio training consisted of 15-min sessions with a couple of minutes rest in between.
The figure below shows that over a period of 20 hours both the Moderate strength training and the Moderate aerobic training boosted the amount of growth hormone in the blood compared with doing nothing. But the increase in these cases was not statistically significant. The increase in the amount of growth hormone in the blood after the Long aerobic training was significant.
The figures above show the amount of growth hormone in the blood of one of the participants. LA = Long aerobic training; MA = Moderate strength training.
The researchers are aware that textbooks and the popular media usually say that it’s strength training that boosts growth hormone secretion and that their study showed unexpected results.
“We believe that the underlying cause may be due to the greater energy expenditure achieved during aerobic exercise and a greater demand for post-exercise fat mobilization”, the researchers write. “The physiological significance of GH pulsatile secretion and its important metabolic role with regard to fuel provision are further reinforced by the recent findings of Surya et al. [J Clin Endocrinol Metab. 2009 Aug;94(8):2828-34.] who reported that only pulsatile, not continuous, delivery of GH stimulates lipolysis in humans.”
20-Hour Growth Hormone Secretory Profiles Following Aerobic and Resistance Exercise.
The pulsatile secretion pattern of growth hormone (GH) is an important parameter of GH action at peripheral tissues and more information is needed on how exercise impacts GH secretion. This study hypothesized that both aerobic and resistance exercise would exhibit dose-response relationships with respect to exercise duration and 20-h post-exercise GH secretion.
Eight healthy men randomly completed five separate, conditions: 1) control (no exercise; CON), 2) a moderate duration (1-hr) aerobic exercise session (MA), 3) a long duration (2-hr) aerobic exercise session (LA), 4) a moderate duration (1-hr) resistance exercise session (MR), and 5) a long duration (2-hr) resistance exercise session (LR). Exercise intensity, diet, sleep and physical activity were strictly controlled during each condition and blood was sampled post-exercise every 20 min for 20 hr and GH secretion parameters were analyzed via cluster and deconvolution analyses.
Only the 2-hr aerobic exercise bout resulted in a significant amplification of GH secretion as evidenced by increases in GH burst peak amplitude (~100%), basal GH secretion rate (~127%), total GH basal secretion (~120%), total pulsatile secretion (~88%), and total GH secretion (~89%) over the control (i.e. no exercise) condition. GH secretion for the resistance exercise conditions were not different from control.
The fact that the 2-hr aerobic exercise condition resulted in higher energy expenditure than the other exercise conditions could offer a partial explanation for the greater GH amplification due to the metabolic effects that GH exerts in stimulating post-exercise lipolysis. We conclude that extending the duration of aerobic exercise, but not resistance exercise, from 1-hr to 2-hr, significantly amplifies GH secretion over a 20-hr period.
PMID: 24576855 [PubMed – as supplied by publisher]